Venus Revisited

New Missions aim to unravel the mysteries of how the planet’s scorching surface and violent clouds came to be.

The first spaceship to pull into Venus orbit in nearly a decade arrived in December 2015, hailing from Japan. Akatsuki was five years late for its rendezvous, but Venus has gotten used to waiting. The European Space Agency’s Venus Express had visited the thickly shrouded world in April 2006, and that was the first mission to Venus since NASA’s Magellan arrived in 1990. Named for the Roman goddess of love, Venus wasn’t feeling much of that from space agencies on Earth. Our planet’s more favored neighbor, Mars, had hosted roughly a dozen visitors in the same period.

“Venus exploration is behind schedule,” said David Grinspoon, senior scientist at the Planetary Science Institute in Washington, D.C. and author of the book Venus Revealed. “Our understanding of Venus is about the same as it was Mars in the 1970s.”

Some [planetary scientists are trying to change that. For years, Venus lost out to Mars because of the tantalizing possibility of finding life on the red planet. Yet in some respects, Venus is more similar to Earth than Mars is, and our inner neighbor might have much to tell us about our past, our future, and even current exoplanets.

Venus wasn’t always so unloved. From 1960 to 1984, some 37 spacecraft investigated Venus — nearly as many as Mars up to that point. The USSR’s Venera and Vega programs resulted in no less than 18 orbiters and landers (though not all missions were successful) and the U.S. added five spacecraft.

Two new NASA missions are in the advanced planning stages, with their fates to be decided this year. Both the ESA and Russian Space Agency have designs on the drawing board. And of course, there’s the current science from Akatsuki finally streaming to Earth. All in all, things are looking up for Venus exploration, and upcoming missions – mostly orbiters but with some plans for landers or craft that will dive into the atmosphere — could answers tfundamental questions that planetary scientists have about Venus and provide hard evidence to nail down their current theories.

The Twin ParadoxVenus is often referred to as a twin of Earth for a reason: the two planets physical properties are nearly identical. “If we found Venus around a sunlike star, [astronomers would] be jumping up and down saying we found another Earth,” says Colin Wilson, deputy project scientist on Venus Express.

How similar are they? Venus has a radius of 3,760 miles (6,051 kilometers) and Earth’s radius is 3,960 miles (6,371km). Venus’ mass is 82 percent that of Earth and surface gravity is 91 percent of the terrestrial norm. The two planet’s densities are also almost identical. That means their bulk composition should be about the same, especially since both planets were formed in the same region of the solar nebula. Their evolution should also have been similar.

But thanks to subtle differences, that didn’t happen. Present-day Earth has liquid water and an atmosphere dominated by nitrogen and oxygen. Argon accounts for nearly 1 percent, but carbon dioxide and other gases exist only in trace amounts. The Venus of today is covered in a dense atmosphere – 90 times more massive than Earth’s — consisting of 97 percent carbon dioxide, with the rest as nitrogen and trace gases. Carbon dioxide a powerful greenhouse gas that keeps surface temperatures average of 864 F (462 C). A visitor to Venus could pour a glass of liquid zinc or lead.

Unlike Earth, Venus’ surface is invisible from above – at least in visible light. It’s covered with highly reflective sulfuric acid clouds that never break. On Venus the sun is a diffuse splotch of brightness, appearing as it does on an overcast day on Earth. That bright patch takes 117 days to cross the sky. Venus takes 243 days to make a complete rotation – longer than the planet’s year, which is 225 days. The daylight period is shortened slightly because the planet rotates retrograde – the Sun rises in the west. The slow rotation also means Venus lacks a magnetic field of any significance.

When it rains on Venus, the droplets evaporate before they reach the ground. Besides a forecast of “cloudy with a chance of sulfuric acid rain” there doesn’t seem to be much in the way of weather at ground level. Surface pressures are so high – 90 plus atmospheres – that it’s just like being underneath more than half a mile (900m) of ocean, and the carbon dioxide there begins to behave as a supercritical fluid, a strange hybrid of liquid and gas.

Previous missions found that Venus’ terrain is as varied as Earth’s. Highland regions called tesserae consist of ridges and folds in the crust that extend for kilometers and form tile-like patterns. The lowlands seem to be basalts, cut with what might be lava channels. Some mountains appear peaked with a kind of metallic “frost,” and even features that look like dune fields exist. Some areas have coronae – pancake features that can spread over 100 miles (160km).

Venus is also bone-dry. If the planet did form with similar amounts of water as Earth, as seems likely, Its clear the water isn’t there anymore.

How did Venus become a toxic hellscape while Earth stayed relatively cool? The prevailing model is that Venus’ water turned to vapor as the sun, which was much dimmer billions of years ago, brightened and warmed the planet. While there’s some debate whether Venus ever shared Earth’s vast oceans, it seems likely the planet was cool enough for substantial liquid water in its early days. But as the temperature climbed, the water evaporated, and once it reached the upper atmosphere the sun’s ultraviolet light broke apart water (H2O) and it quickly reformed into hydrogen (H2), hydroxide (OH) and oxygen (O2). Much of the oxygen stayed aloft because it is less dense than carbon dioxide, but some descended and reacted with surface rocks. Absent any biology to take the CO2 out of the air and replace it with oxygen, as on Earth, the water and carbon dioxide — powerful heat-trapping gases – caused a runaway greenhouse effect.

The situation wasn’t helped by the planet’s slow rotation. On Earth, the relatively rapid spin creates a dynamo effect in the planet’s iron core. This in turn generates a powerful magnetic field that protects our home world from the solar wind, the stream of energetic particles that the Sun flings in all directions. As the wind whipped by unprotected Venus, it stripped the hydrogen from the atmosphere, leaving fewer ingredients for the planet to have any hope of reforming its water, even if conditions were miraculously to become more temperate.

Yet the data from Venus Express, and from the Magellan probe seem to tell the whole story – and that’s where the new missions come in.

Many Mission Options

NASA is considering two missions this year. The Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging (DAVINCI), will focus on atmospheric chemistry. It involves an atmospheric probe – “Huygens for Venus,” quips Lori Glaze, of NASA’s Goddard Space Flight Center and the mission’s principal investigator – that will measure the atmosphere’s makeup at different layers during an hourlong descent to the Venusian surface.

“It doesn’t need to survive hitting the surface,” Glaze says. She notes Pioneer Venus and the Vega missions looked at the atmosphere, but they couldn’t give scientists a good handle on the composition with respect to altitude, and that’s needed to understand what kinds of reactions occur in Venus’ cloudy skies.

NASA’s other option is the Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy (VERITAS) mission. The VERITAS orbiter would operate similar to Magellan, but the big difference will be that the radars will have much better resolution, able to spot features 100 feet (30m) across compared with Magellan’s more than 300 feet (100m). It also will measure the planet’s gravity and how the surface emits heat, which means it can see “inside” some geological formations, and discover, for example, whether the coronae are filled with magma. VERITAS will also be able to measure how the composition of surface rocks differs.

“We’ll be looking for surface mineralogy variations,” said Suzanne Smrekar of the Jet Propulsion Laboratory, the principal investigator on VERITAS. “We’re trying to understand chemical variation, if there are continents like on Earth, active volcanism… Also to see if there are tectonic features, and to try to understand thermal evolution — temperature variations in the lithosphere.” NASA will decide in September 2016 whether DAVINCI, VERITAS, or both will fly.

From the ESA, there’s EnVision. EnVision is an orbiter also equipped with advanced radar, and it likely won’t launch until 2029, says Richard Ghail, a lecturer in engineering geology at Imperial College, London, who proposed the mission. Aside from a more advanced set of radars than Magellan, EnVision would have the ability to “spotlight” small areas to image them in greater detail. “Instead of 100 meter [300 feet] resolution we can get down to 6 meters [20 feet],” Ghail says. That is enough to see day-by-day changes in the surface. Spotlighting can get that resolution down to 10 feet (3m).

While there is some overlap with a craft like VERITAS, Ghail said having the VERITAS mission go would actually free up EnVision to do more spotlighting specific areas and less global scale mapping, since VERITAS will have accomplished that already.

The Russian proposal is called Venera D. Venera D would bear some resemblance to the Vega missions, because it involves a combined orbiter and lander. It might even include a balloon probe, also like the Vega missions. Like many missions before it, Venera D would focus on the planet’s atmosphere and investigate the origins of Venus’ unusual atmospheric rotation, as well as its chemistry. The lander would allow for soil analysis, provided it lasts long enough. The longest any lander has lasted on Venus was barely two hours –a record held by the Soviet Venera 13 mission, so history is on Russia’s side. The Russian Space Agency hasn’t made any firm commitments to the mission, but if it did it would be the first post-Soviet planetary mission of its kind. Launch wouldn’t happen any earlier than 2024.

Venus NowCurrent Venus research is already on a delayed schedule. Japan’s Akatsuki probe was supposed to arrive at Venus in 2010, but it missed its orbital insertion. Instead, it took a cruise around the Sun for five years until engineers could steer it into an alternate orbit, more elliptical than originally planned. Recoverring the spacecraft at all was a major achievement. Akatsuki currently orbits Venus in a 13-day ellipse that takes it from a closest approach of 260 miles (400km) out to 273,000 miles (440,000km).

Sanjay Limaye, senior scientist at the Space Science and Engineering Center of the University of Wisconsin-Madison, says science goals will actually be enhanced by the improvised orbit, which allows for longer periods of observation as Akatsuki swings out to its farthest position.

Two of Akatsuki’s cameras work in the near-infrared and study the planet’s surface, the motion of clouds, and the particles that comprise them. A long-wave infrared camera tracks the temperatures at the cloud tops, about 40 miles (65km) above the planet’s surface. The other two instruments are an ultraviolet imager and a lightning and airglow camera. Venus Express showed tantalizing glimpses of what might have been lighting in Venus’ clouds, so Akatsuki will try to clinch those observations.

One of the problems Akatsuki will study is the “superrotation” of the atmosphere. Venus’ atmosphere zooms around the planet at hundreds of kilometers per hour in the upper regions. That’s not unusual – other planets show the same thing from time to time. But why the superrotation should be orders of magnitude faster than the rotation is as yet unexplained. “We cannot yet accurately model superrotation numerically,” Lamaye said. Akatsuki can help tackle this question by giving a better picture of how the upper atmosphere differs from the lower and how the two interact.

A Smoking Gun For VolcanismVenus Express and Pioneer Venus both found sulfur compounds – primarily sulfur dioxide, which must continually enter the atmosphere somehow in order to be observed, because sunlight breaks it up fairly quickly. “That provides pretty good evidence that Venus is volcanically active, Grinspoon said. “There’s a lot of sulfur dioxide in the atmosphere, and that sulfur would not stay without a source.” Volcanism on the surface would do it. But Venus Express hasn’t provided the smoking gun as it were. “We see a lot of volcanoes but we don’t know if they are still active,” Grinspoon adds.

There are three strong lines of evidence of active volcanoes on Venus, Wilson says. First is the way the detected sulfur behaves. In the first year of Venus Express’ observations sulfur levels spiked,and then decreased by tenfold over five to six years. That points to a source that “burped” up sulfur, as volcanoes do. A second clue is the infrared surface emission. Darker surfaces emit more heat as infrared radiation (think about asphalt on a hot day) and fresh unweathered basalts – such as from recently spewed volcanic material — are dark. Venus Express’ cameras also caught some changes in surface temperature that looked like signs of recent lava flows. Finally, images from Magellan’s radar maps show features that look pretty clearly like volcanoes, and even lava.

But none of that is absolute proof, Glaze says. What’s needed is a picture from one day to the next, or one week to the next, showing the changes in topography. That could show that volcanoes are active today, as opposed to in the distant past.

Volcanic activity is such a big piece of the overall Venus puzzle because it offers a way to resurface the planet periodically. Previous imaging missions showed Venus doesn’t have many impact craters. Assuming the craters are randomly distributed, that means something made older ones disappear – the surface is getting rebuilt every so often, perhaps as little as every couple of hundred million years or as much as 750 million to 800 million years. And given that the craters are on average hundreds of miles apart, whatever is resurfacing the planet must also exist in the same well-distributed patern, Ghail says.

If there are active volcanoes – and Ghail thinks there are — then this resurfacing is a constant, steady process. But if volcanoes aren’t currently active then the resurfacing is likely to be something big and sudden, covering huge chunks of the planet. VERITAS and EnVision could both go a long way toward providing a clear answer.

Where’s The Water?The other big question mark is water. Venus Express’ atmospheric analyses show the ratio of deuterium (hydrogen that carries an extra neutron) to ordinary hydrogen is quite large, and that hydroxide is in the atmosphere. Ordinary hydrogen is lighter than deuterium, and a high ratio of deuterium would mean the primordial hydrogen was stripped away somehow, probably by the solar wind. The hydroxide is a product of the dissociation, or chemical breakup, of water by ultraviolet light. But Venus surprised the scientists. “We’d expect it to lose water faster,” says Wilson. “But the escape rate is less than on Earth. That came as a surprise.” Further atmospheric and geological studies might shed some light on this, by narrowing down the rate of outgassing water from the surface, for example.

Speaking of water, geological tests by VERITAS can help scientists understand how much water Venus has now. Such tests could reveal if Venus once had something like plate tectonics, or formed a surface resembling that of early Earth. By looking closely at what kind of rock makes up some of the higher-altitude terrain, such as the tesserae, it will be possible to see if they are made of crust that looks like continents on Earth. “That is critical to answering the question,” Smrekar says. “What you’re measuring is surface temperature in relation to the composition of rock. Basalt is a dark rock, granite is a light rock, and they have different temperatures as a function of altitude.”

To make granite, you need water. “If you don’t have water you end up with things that approach granite but never get that far,” Ghail says. Finding granite, therefore, would mean Venus had oceans – or at least enough water to allow for the reactions that make granite.

Ghail says the way the higher terrain looks — such as Aphrodite, Lakshmi and Ishtar Terra — is tantalizing. “Aphrodite looks like ancient heavily formed continental like material,” he says. Furthermore, these regions seem to cover about the same area that geologists think was covered by continental crusts on Earth soon after it formed and the oceans first filled up.

Knowing what Venus was like in the distant past will offer a lot of insight into why Earth’s twin grew up so different from its temperate sister. “Venus in my mind is an incredibly rich place to learn about Earth,” Glaze says. “The planets are so similar – how did biology form here and not there?”